The "direct chlorination of ethylene" is the basis for the widely used commercial catalytic process for the production of ethylene dichloride ("EDC", or 1,2-dichloroethane). The reaction is controlled by mass transfer, with absorption of ethylene as the limiting factor whether the reaction is carried out with a slight excess of ethylene, or as an alternative option (considerations relating to which are set forth hereinafter), a slight excess of chlorine, fed to the reactor. The heat of reaction is dissipated either through conventional water cooling of a typical low temperature direct chlorination reactor operating in the range from about 50.degree. C. to about 65.degree. C., or by operating the reactor at, or near, the boiling point of EDC under pressure up to about 200 psig, hence referred to as a "high temperature direct chlorination (HTDC) reactor". The HTDC reactor is a particular type of direct chlorination reactor. In one embodiment, referred to as a "boiling reactor" the HTDC is operated at the boiling point of EDC, and product EDC is drawn off as vapor; in another embodiment, referred to as a " non-boiling reactor", the HTDC is operated near the boiling point and product EDC is drawn off as a liquid sidestream.
The direct chlorination reaction may be written: EQU CH.sub.2 .dbd.CH.sub.2 +Cl.sub.2 .fwdarw.ClCH.sub.2 CH.sub.2 Cl
and theoretically, neither water nor HCl is formed as a product of this reaction. In practice, in the presence of oxygen, some water may be formed in some side reactions, and some HCl is formed in another side reaction which may be written: EQU ClCH.sub.2 CH.sub.2 Cl+Cl.sub.2 .fwdarw.ClCH.sub.2 CHCl.sub.2 +HCl
The precise amount of HCl formed depends upon the type of catalyst used in the HTDC reactor, the liquid medium in which the reaction is carried out (typically a chlorinated hydrocarbon such as EDC), and the conditions of reaction.
The direct chlorination process is desirably complemented by an oxychlorination ("oxy") process in which ethylene reacts with HCl and oxygen to produce EDC in an oxy reactor. This combination of direct chlorination and oxychlorination processes is referred to as "the balanced process" (for further details see the chapter titled "Vinyl Polymers (Vinyl Chloride)" by Cowfer, J. A. and Magistro, A. J, Encylcopedia of Chemical Technology, Kirk & Othmer, Vol 23, 865-885). In the flowsheet therein, it was there suggested that crude EDC produced in the HTDC reactor be neutralized with alkali. The obvious economic burden of disposing of the neutralized material added to the cost of alkali, dictates that this be a less preferred solution.
It has long been known that the effluent from a HTDC reactor is highly corrosive. Recently it was found that the main cause of such corrosion is the presence of free chlorine and trace quantities, less than 100 parts per million (ppm), of water. A process for scavenging free chlorine in an EDC stream, to minimize the corrosion due to the chlorine, is disclosed in U.S. Pat. No. 4,547,599. This corrosion problem is aggravated when the chlorine feed to the boiling reactor is "wet", that is, contains at least 100 ppm of water, which, for example, is the case with gaseous chlorine from electrolytic cells. This problem also arises in a recycle line, including a vent compressor and related equipment, to the oxy reactor, when vent gases vented after recovery of product EDC, are recycled to the oxy reactor. It stands to reason that if there is no water being introduced in the feed to the HTDC reactor, and no water is generated in the direct chlorination reaction, there will be no water in the effluent from the reactor, and no corrosion problem to be solved.
As is well known, the economics of chemical engineering unit operations in the production of EDC are such that, optimally, the ethylene and chlorine are converted to EDC without the formation of unwanted byproducts and most important, without leaving any free chlorine in the effluent. The problem of corrosion is discussed in "Alloy Selection for VCM Plants" by Schillmoller, C. M., Hydrocarbon Processing pg 89-93, March 1979.
In practice, economics dictate that the direct chlorination reaction be controlled so that carbon steel equipment may be used. The problem is that free chlorine and water in carbon steel equipment and piping has a highly corrosive effect far more deleterious than either one or the other, and as little as from about 20 ppm to about 60 ppm of chlorine with trace amounts of moisture in the range from 10 ppm to about 50 ppm upstream of the EDC reactor, will destroy its tubes. The corrosion is exacerbated by the injection of oxygen into the direct chlorination reactor, for reasons set forth hereinfter.
For the foregoing reason, the only practical option is not to use an excess of chlorine in the reactor thus minimizing the amount of unreacted chlorine (referred to as "free" or "breakthrough" chlorine) leaving the reactor; instead, an excess of ethylene is supplied to the reactor. By "excess" ethylene I refer to an amount greater than that stoichiometrically required to produce the EDC, and typically from 1 to about 5% excess may be used, less than 2% excess being preferred. However, even when more than a 2% excess ethylene is supplied to minimize unreacted chlorine, the amount of free chlorine in the effluent remains in the range from about 100 ppm to about 3000 ppm, and substantially all of it has to be removed before the EDC is converted to VC monomer. It is economically onerous to use much more than a 2% excess of ethylene, but even doing so, then attempting to scavenge unreacted chlorine by injecting ethylene into the effluent, does not eliminate the chlorine. The excess ethylene used gets vented as a "vent stream" during recovery of product EDC and is recycled, usually to the oxychlorination reactor along with such moisture, chlorine and HCl as may be present.
The EDC is purified, then pyrolyzed in an EDC cracking furnace to produce vinyl chloride monomer ("VCM") in a reaction referred to as dehydrochlorination, the details of which are well known, and HCl generated in the furnace is recycled to the oxychlorination reactor.
The very small amounts of moisture, chlorine and HCl in the vent stream, each of which is present in relatively small amounts of the vent stream the major portion of which is ethylene and nitrogen, do not appear to be worth recovering because the cost of recovery due to severe corrosion problems, would outweigh the value of the recovered components. But the value of removing moisture to minimize corrosion of the equipment in the recycle line including equipment, to the oxy reactor, which value was never realized in the prior art, with the added value of ethylene and HCl recovered for recycle to the oxy reactor, justifies the cost of recovery.
In the prior art, the goal in a balanced process was the recovery and recycling of ethylene, chlorine and HCl in the effluent from any available source, whether direct chlorination reactor, condensers, storage tanks, and the like. And, as will readily be apparent if such a combined effluent is to be recovered for its chlorine, HCl and ethylene values, it is logical to recycle it to the oxy reactor. The major emphasis was on the recovery of ethylene which they used in large excess to minimize the amount of unreacted chlorine, and they appear to have been unconcerned with the effect of moisture on the materials of their equipment, as they did not dry the vent stream they recycled.
Such a process for the recovery of combined vent gases containing ethylene, chlorine, HCl and water, which gases are generated in an EDC plant, is disclosed in Offenlegungsschrift DE No. 3044854 Al published July 1, 1982. The vent gases from a direct chlorination reactor operating at atmospheric pressure or above, are cooled to a temperature in the range from 1.degree. to 2.degree. C., but no cooler, so that the water in the vent gases does not freeze and plug up the lines. The vent gases which do not condense are then washed with water and alkali to remove unreacted chlorine. Clearly they had no intention of removing water, and of course, condensed only so much as the partial pressure of water in the vent stream would allow at a temperature above the freezing point of water.
The reference also teaches that attempts to remove a higher ratio of condensables by dropping the temperature to -20.degree. C. were unsuccessful because the moisture present in the lines froze and plugged them. It was this discovery which led the German patentees to cool the vent gases to above the freezing point of water, and tolerate the smaller ratio of condensables including water, which they obtained at the higher condensing temperature since they were interested in recycling the combined vent stream to the oxychlorination reactor where the presence of additional moisture was not material. As is well known, an equimolar amount of water and EDC is generated in the oxychlorination reaction which may be written as follows: EQU CH.sub.2 .dbd.CH.sub.2 +2HCl+0.50.sub.2 .fwdarw.ClCH.sub.2 CH.sub.2 Cl+H.sub.2 O
For the patentees, water was not removed, and in the particular instance in the prior art referred to by the patentees, where the vent gases were chilled to the subfreezing temperature (of water), it is evident that the formation of ice (which plugged the lines and equipment) defeated the removal of water on a continuing basis. Thus, such separation as may have occurred was incidental or accidental and had nothing to do with minimizing the corrosion in the equipment due to the presence of water in the vent gases. Most of all, it may not have been realized that isolating the vent gases from the product column, avoided the problem of too much water in the combined vent gases from all over the EDC facility. Not coincidentally, the choice of the materials of construction of their recycle line and equipment appears to have been made to cope with the problem of corrosion due to the presence of the chlorine and moisture, both in the effluent line from the HTDC reactor, and in the recycle line to the oxy reactor.